1. Field of the Invention
The present invention relates to a process for chemically metallizing an inorganic substrate which is poor conductor of electricity and, in particular, to a wet chemical process of the electroless and/or electrolytic type for providing a copper metal layer on an inorganic substrate composed of a ceramic material, glass, enamel or other oxidic, nitridic, carbidic, silicic, boridic material or of mixtures or mixed compounds of these inorganic materials.
2. Background of the Art
Oxide ceramic materials, such as aluminum oxide, beryllium oxide, ferrite, titanate, zirconate, as well as quartz, enamel and some glasses, are often used in the electrical and electronic arts. The foregoing exemplary materials are basic substrate materials used in the electrical and electronic arts for film-type circuits (thick film circuits, thin film circuits, special circuit boards, hybrid circuits, and the like), carrier elements for discrete components (capacitors, filters, oscillators, resonators, integrated circuits, and the like), components for transmitting tubes, microwave tubes, and transparent front plates for displays provided with electrical supply or actuation lines.
Other ceramic or glass materials are used as assembly elements, carrier elements or in the form of layers in the semiconductor art. Examples of materials for these applications are silicon nitride, silicon oxide nitride, as well as carbidic layers. Quartz glass and other glasses are used as light waveguides in the laser art and in the optical communications art. Beryllium oxide is appreciated for its favorable heat conducting capability in components subject to thermal stresses.
In order to be functional in these electrical or electronic applications, these materials or layers must be metallized for the production of conductive paths and conductive terminals. Metallization is generally accomplished with copper or a copper alloy, since copper meets the requirements particularly well with respect to electrical conductivity, ductility, and solderability. Further, copper and copper alloys can be deposited in an electroless, i.e., a currentless chemical manner and can be then electrolytically reinforced using the layer deposited chemically as an electrode.
However, the adhesion of the copper coating to the substrate material generally poses a difficult problem since the substrate is usually chemically inert, very smooth, and repels surface coatings. If, for electrical or optical reasons, for example, the surface of the substrate material cannot be roughened or can only be roughened slightly, mechanical anchoring of the metal layer to the substrate contributes little to adhesion. Under such circumstances, a very thin copper layer readily develops bubbles during its electroless precipitation from a chemical copper bath and easily chips and peels away from the substrate. At the same time, however, wet chemical deposition is a preferred metallization technique since this procedure is essentially independent of format and geometry of the workpiece. In contrast to screen printing, vapor deposition or sputtering techniques, wet chemical deposition, i.e., electroless and/or electrolytic precipitation, is capable of precipitating a metal layer onto various structural forms or geometries, such as throughbores and other edges, essentially without problems.
In order to solve the adhesion problem attendant to wet chemical metallization, various prior art processes are known which employ an adhesion-promoting, intermediate layer. For example, German Pat. No. 2,004,133, which corresponds to U.S. Pat. No. 3,802,907 to Roland Apfelbach et al, discloses a process which provides a silicic acid or silicate intermediate layer; German Pat. No. 2,533,524, a copper oxide intermediate layer; and German Pat. No. 2,453,192, an acid or alkali adhesion promoter which is matched to the acidity of the substrate material. The mechanism by which these adhesion promoters improve adhesion is thought to be the fact that they form chemical bonds with the substrate material on the one hand and the copper coating on the other hand. These bonds are believed to be developed by mechanisms including dehydration, solid-state diffusion and/or solid-state reactions, since cotemporaneous thermal treatment or subsequent thermal treatment favorably influences the kinetics of these mechanisms and, thus, adhesion of the resulting metallization.
In principle, the attainable adhesive strength, measured as vertical peel strength, is very high and may even exceed the tensile strength of ceramic substrate itself. However, these high adhesive strengths are generally not realized uniformly over the entire substrate surface, but only at certain surface loci, which loci are believed to be particularly active centers present on the surface of the substrate. The same non-uniformity in adhesive strength is noted for the edges and throughbores of the substrate. This non-uniform effect, with its inherent disadvantage of causing production rejects, occurs most frequently in connection with chemically inert substrate materials which undergo a high temperature process during manufacture, such as sintering, and receive a firing skin or otherwise have been "killed by heat" (rendered chemically inert) during the sintering process. In such materials, the usual surface cleaning, for example, with organic solvents, with diluted aqueous cleaning agents based on a surfactant, or with chromosulfuric acid, is not sufficient.